70 HENRY BURY. 
be readily seen, is at right angles to the plane of the water- 
tube. 
This obliquity of the mesentery causes the right body-cavity 
to appear, in sections through the extreme posterior end of the 
larva, somewhat smaller than the left; but this is not the case 
in sections through more anterior regions (fig. 20), and indeed 
the total bulk of the right posterior body-cavity (deducting the 
anterior body-cavity, which may be said to end near the 
anterior margin of the stomach) is, at the time of the formation 
of the water-vascular ring, decidedly greater than that of 
the left posterior cavity. 
The later stages of development do not call for any detailed 
description in this paper. After the junction of the radial 
tubes of the hydroccel with the excrescences of the body-wall 
which contain the terminals, the two grow out together and 
form the arms, into which, in the larva, only the left body- 
cavity extends. By this means, as well as by increased growth 
within the disc, the left body-cavity gradually exceeds the right 
in size, though even in the oldest larva I have obtained (with 
fifteen pairs of tube-feet to each arm, and a total diameter of 
3-9 mm.) there is still, within the disc alone, not very much dif- 
ference between them (fig. 23). 
In this latest stage the former protuberance of the stomach 
in the centre of the water-vascular ring has been pushed back 
by the formation of an ectodermic stomodeum ; but the adult 
cesophagus is not yet complete. The stomach is bound to the 
body-wall not only by the mesentery, but also by five interra- 
dial septa, one of which, as we saw, forms one wall of the 
axial sinus. Radially the stomach has five pairs of pouches, 
as seen in fig. 24, and the relation of these to the mesentery is 
shown in section in fig. 25; as will be seen, the right body- 
cavity is dorsal to the stomach, no portion of it being visible 
in fig. 24. The anus is, as shown, still in the same interradius 
as the water-tube. As I have never succeeded in rearing a post- 
larval specimen from this Bipinnaria, I am quite unable to 
trace the migration of the anus into the position which it occu- 
pies in the adult; but it is probable that the larval anus is 
